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The Franklin News-Post
P. O. Box 250
310 Main Street, SW
Rocky Mount, Virginia 24151
Fax: 540-483-8013

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Blight infecting county’s tomato gardens
Once disease is present, it’s usually too late
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Photo from This is an image of late blight disease on the leaves of a tomato plant.

Friday, August 16, 2013


The majority of Franklin County's gardens are now infected with late blight, according to Extension Agent Sean Duff.

That judgment is based the number of calls and visits from Franklin County residents and the number of tissue samples being sent for analysis, said Duff.

Late blight is a disease that begins as small, water-soaked lesions on leaves closest to the ground and continues up the plant, developing into brown, oily necrotic leaf tissue.

After the leaf has died, the infection begins to spread to the fruit in the form of large brown spots with a leathery feel and appearance.

Late blight of potato and tomato plants is caused by Phytophthora infestans, an organism that superficially resembles a fungus, but is more closely related to the brown algae.

According to the website,, the disease led to the Irish potato famine in 1845, and under the right weather conditions, tomato and potato crops can be destroyed within days.

"It is a devastating disease, but by no means should that raise a concern," said Duff. "We have been coping with this blight for many years, utilizing knowledge of plant breeding and advances in chemical controls."

Wet, cool environmental conditions favor development of late blight, said Elizabeth Bush, plant pathologist with the Virginia Cooperative Extension.

"During wet, cool weather, crop loss due to late blight

can be rapid and unstoppable if preventative controls

have not been used," she said.

The disease thrives in medium to high humidity (70-90 percent) and moderate temperatures (60-85 degrees F) and is spread from plant to plant by wind and rain, Duff said.

"The disease is said to overwinter in tomato and potato stubble from the previous growing season and the soil surrounding those plants," said Duff.

Duff suggests anyone with late blight in their gardens, start off their plan of attack by removing infected plants, including the roots, at the end of the growing season this year.

Next year, begin by planting varieties of potatoes and tomatoes that are less susceptible to late blight.

Frequent, low dose applications of preventative fungicides containing the active ingredient Chlorothalonil can help alleviate disease incidence and transmission prior to infection, Duff said.

He also suggests avoiding the overhead irrigation of plants.

"Water the plants as close to the soil as possible to keep the foliage dry," said Duff.

Bush also recommends reducing leaf wetness by spacing plants adequately, limiting the size of the

plant canopy, irrigating in the morning to promote foliar drying, staking plants and planting in locations with good air movement.

"Avoid excessive use of nitrogen fertilizer," said Bush. "It promotes succulent tissue that is more prone to infection."

Those who suspect late blight in their garden, or wish to ask questions about late blight, should contact the extension office at (540) 483-5161.

"A grower who waits until the disease is present in a crop to begin control tactics generally will be fighting a losing battle," said Bush.

More information about late blight can be found at

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